An additional and important advantage of the polyethylene composition of the present invention is that it can be melt-processed at unusually high shear rate values, which mean high processing speeds and/or reduced melt-processing temperatures, without encountering flow-instabilities which generally produce unacceptable defects in the formed articles (e.g. shark skin or melt-fracture), even in the absence of processing aids.
Moreover, the fast crystallization kinetics of the present composition, which provides a critical contribution to its superior process-ability, also provides an unusually reduced shrinkage of the formed articles, thus allowing achieving a remarkable dimensional stability. Thus the composition of the present invention provides an unmatched balance of mechanical properties and process-ability with respect to the known polyethylene compositions for the same use, as disclosed in particular in U.S. Pat. No. 6,201,078.
In fact, the polymers disclosed in U.S. Pat. No. 6,201,078 achieve a relatively low balance of swell ratio and environmental stress cracking resistance, as shown in the examples.
The problem of achieving a high impact resistance, reducing the flow-instabilities and improving the dimensional stability (lowering shrinkage) is not mentioned in such document.